The invention pertains to machine vision and, more particularly, to methods for calibrating the imaging reference frame of a camera (or other image acquisition device) to that of a moveable object imaged by the camera.
Machine vision is the automated analysis of images to determine characteristics of objects shown in them. It is often employed in automated manufacturing lines, where images of components are analyzed to determine placement and alignment during assembly. Machine vision is also used for quality assurance. For example, in the semiconductor device industry, images of chips are analyzed to insure that leads, solder paste and other components do not overrun designated boundaries.
In many machine vision applications, it is essential to correlate physical coordinates in the "real world" with pixel coordinates in an image. For example, a camera image of a circuit board being assembled on a motion stage--a motorized platform that can be positioned by rotation and by movement along the x- and y-axes--may reveal that a chip or other component is misplaced by several pixels. In order to properly reposition the stage, the precise relationship between coordinates in the image and those of the stage must be known.
That relationship is known as the calibration relationship. It includes the angular orientation of the reference frame of the image vis-a-vis the reference frame of motion of the stage; the ratio of a unit of height (i.e., pixel height) in the image reference frame to a unit of motion along the y-axis of the stage; the ratio of a unit of width (i.e., pixel width) in the image reference frame to a unit of motion along the x-axis of the stage; and the location of a point in the image reference frame (e.g., the center of the camera field of view) vis-a-vis a point in the motion reference frame (e.g., the center of rotation of the stage).
The prior art suggests the use of calibration plates to determine the relationship between the imaging and motion reference frames. The surfaces of these plates are typically machined or etched with precisely spaced markings such as checkerboards, "bulls-eyes," or other such patterns. When camera images of the plate and stage are analyzed, the known spacings between markings on the plate are compared with those that appear in the images, thus, enabling calculation of the calibration relationship.
It is important to fabricate calibration plates carefully, because any imperfections in the marking pattern can lead to inaccurate calibrations. Unfortunately, even a precisely machined calibration plate may not be sufficiently accurate for use in calibrating highly magnified images. Small deviations falling well within tight machine tolerances may adversely affect the calibration relationship. This problem is further exacerbated in machine vision systems that use multiple cameras to image a single object. Such systems are used, for example, in semiconductor chip manufacture, where two or more high-resolution cameras are used to simultaneously inspect multiple regions on the chip surface. In addition to the difficulties associated with calibrating the reference frame of a single camera to the real world reference frame of the chip surface (or motion stage) are those associated with calibrating the reference frames of the cameras to one another.
An object of this invention is to provide improved machine vision systems and, particularly, improved machine visions methods and apparatus for calibrating the imaging reference frames to multiple cameras that generate images of a common object.
Another object of the invention is to provide machine vision methods and apparatus for calibrating the imaging reference frames to multiple cameras that generate images of objects on a common motion stage or conveyor, or that otherwise move with one another.
A still more particular object of the invention is to provide such methods and apparatus as are capable of providing accurate calibrations without the use of expensive calibration plates.
Yet another object of the invention is to provide such methods and apparatus as can be implemented on conventional digital data processors or other conventional machine vision analysis equipment.
Yet still another object of the invention is to provide such methods and apparatus that can rapidly determine calibration relationships quickly and without undue consumption of resources.